UPD70741 datasheet(18/112 Pages) NEC

18 / 112 page

µPD70741

3.3.1 Program register set

The program register set includes general-purpose registers and a program counter.

(1) General-purpose registers

The V821 has 32 general-purpose registers, r0-r31. These registers can be used for data or address variables.

Registers r0 and r26-r30 are used implicitly with instructions. Caution is therefore necessary when using these

registers. Registers r1-r5 and r31 are used implicitly by the assembler and the C compiler. Before using these

registers, therefore, the contents of the registers must be saved to prevent their being destroyed. After using

the registers, their contents must be restored.

Table 3-1. Program Registers

Name

Use

Explanation

Zero register

Always stores zeros.

Assembler-reserved register

Used as a working register to create 32-bit immediate.

Handler stack pointer

Used as a stack pointer for the handler.

Stack pointer

Used to create a stack frame at a function call.

Global pointer

Used to access a global variable in a data area.

Text pointer

Points to the top of a text area

r6-r25

r26

String destination start bit offset

Used to execute a bit string instruction.

r27

String source start bit offset

r28

String length register

r29

String destination start address register

r30

String start address register

r31

Link pointer

Stores a return point address according to the execution of a JAL instruction.

(2) Program counter

Stores the address of an instruction while a program is running. Bit 0 of the program counter (PC) is fixed to

0, thus preventing a branch to an odd address. It is initialized to FFFFFFF0H at reset.

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